Glass compositions having high thermal and chemical stability and methods of making thereof

ABSTRACT

Described herein are alkali-free, boroalumino silicate glasses exhibiting desirable physical and chemical properties for use as substrates in flat panel display devices, such as, active matrix liquid crystal displays (AMLCDs). In accordance with certain of its aspects, the glasses possess good dimensional stability as a function of temperature.

CROSS-REFERENCE TO RELATED APPLICATION

This application is a continuation of: (1) pending U.S. application Ser.No. 12/943,268 filed on Nov. 10, 2010, which is a divisional of U.S.application Ser. No. 11/704,837 filed on Feb. 9, 2007, now U.S. Pat. No.7,833,919, which claims the benefit under 35 USC §119(e) of U.S.Provisional Application No. 60/772,600 filed Feb. 10, 2006; and (2)pending U.S. application Ser. No. 13/666,183 filed on Nov. 1, 2012,which is a continuation of pending U.S. application Ser. No. 12/943,268.The contents of U.S. application Ser. Nos. 13/666,183, 12/943,268,11/704,837, and 60/772,600 are hereby incorporated by reference in theirentireties.

BACKGROUND

The production of liquid crystal displays such as, for example, activematrix liquid crystal display devices (AMLCDs) is very complex, and theproperties of the substrate glass are extremely important. First andforemost, the glass substrates used in the production of AMLCD devicesneed to have their physical dimensions tightly controlled. The downdrawsheet drawing processes and, in particular, the fusion process describedin U.S. Pat. Nos. 3,338,696 and 3,682,609, both to Dockerty, are capableof producing glass sheets that can be used as substrates withoutrequiring costly post-forming finishing operations such as lapping andpolishing. Unfortunately, the fusion process places rather severerestrictions on the glass properties, which require relatively highliquidus viscosities.

In the liquid crystal display field, thin film transistors (TFTs) basedon poly-crystalline silicon are preferred because of their ability totransport electrons more effectively. Poly-crystalline based silicontransistors (p-Si) are characterized as having a higher mobility thanthose based on amorphous-silicon based transistors (a-Si). This allowsthe manufacture of smaller and faster transistors, which ultimatelyproduces brighter and faster displays.

One problem with p-Si based transistors is that their manufacturerequires higher process temperatures than those employed in themanufacture of a-Si transistors. These temperatures range from 450° C.to 600° C. compared to the 350° C. peak temperatures employed in themanufacture of a-Si transistors. At these temperatures, most AMLCD glasssubstrates undergo a process known as compaction. Compaction, alsoreferred to as thermal stability or dimensional change, is anirreversible dimensional change (shrinkage) in the glass substrate dueto changes in the glass' fictive temperature. “Fictive temperature” is aconcept used to indicate the structural state of a glass. Glass that iscooled quickly from a high temperature is said to have a higher fictivetemperature because of the “frozen in” higher temperature structure.Glass that is cooled more slowly, or that is annealed by holding for atime near its annealing point, is said to have a lower fictivetemperature.

The magnitude of compaction depends both on the process by which a glassis made and the viscoelastic properties of the glass. In the floatprocess for producing sheet products from glass, the glass sheet iscooled relatively slowly from the melt and, thus, “freezes in” acomparatively low temperature structure into the glass. The fusionprocess, by contrast, results in very rapid quenching of the glass sheetfrom the melt, and freezes in a comparatively high temperaturestructure. As a result, a glass produced by the float process mayundergo less compaction when compared to glass produced by the fusionprocess, since the driving force for compaction is the differencebetween the fictive temperature and the process temperature experiencedby the glass during compaction. Thus, it would be desirable to minimizethe level of compaction in a glass substrate that is produced by adowndraw process.

There are two approaches to minimize compaction in glass. The first isto thermally pretreat the glass to create a fictive temperature similarto the one the glass will experience during the p-Si TFT manufacture.However there are several difficulties with this approach. First, themultiple heating steps employed during the p-Si TFT manufacture createslightly different fictive temperatures in the glass that cannot befully compensated for by this pretreatment. Second, the thermalstability of the glass becomes closely linked to the details of the p-SiTFT manufacture, which could mean different pretreatments for differentend-users. Finally, pretreatment adds to processing costs andcomplexity.

Another approach is to slow the kinetics of the compaction response.This can be accomplished by raising the viscosity of the glass. Thus, ifthe strain point of the glass is much greater than the processtemperatures to be encountered (e.g., if the strain point is ˜200-300°C. greater than the process temperatures for short exposures),compaction is minimal. The challenge with this approach, however, is theproduction of high strain point glass that is cost effective.

Described herein are alkali-free glasses and methods for making the samethat possess high strain points and, thus, good dimensional stability(i.e., low compaction). Additionally, the glass compositions alsopossess all of the properties required for downdraw processing, which isimportant in the manufacturing of substrates for liquid crystaldisplays.

SUMMARY

In accordance with the purposes discussed above, the disclosedmaterials, compounds, compositions, articles, devices, and methods, asembodied and broadly described herein, are alkali-free, boroaluminosilicate glasses exhibiting desirable physical and chemical propertiesfor use as substrates in flat panel display devices, such as, activematrix liquid crystal displays (AMLCDs). In accordance with certain ofits aspects, the glasses possess good dimensional stability as afunction of strain point. Additional advantages will be set forth inpart in the description that follows, and in part will be evident fromthe description, or may be learned by practice of the aspects describedbelow. The advantages described below will be realized and attained bymeans of the elements and combinations particularly pointed out in theappended claims. It is to be understood that both the foregoing generaldescription and the following detailed description are exemplary andexplanatory only and are not restrictive.

BRIEF DESCRIPTION OF THE DRAWINGS

The accompanying figures, which are incorporated in and constitute apart of this specification, illustrate several aspects described below.

FIG. 1 shows the dependence of dimensional change, here labeled“compaction,” versus strain point for a series of glass compositionsdescribed herein heated at 600° C. for five minutes.

FIG. 2 shows the compaction behavior versus time of three glass samplesgiven repeated heat treatments at a temperature of 600° C.

DETAILED DESCRIPTION

The materials, compounds, compositions, articles, devices, and methodsdescribed herein may be understood more readily by reference to thefollowing detailed description of specific aspects of the disclosedsubject matter and the Examples included therein and to the Figures.

Before the present materials, compounds, compositions, articles,devices, and methods are disclosed and described, it is to be understoodthat the aspects described below are not limited to specific syntheticmethods or specific reagents (specific batch components), as such may,of course, vary. It is also to be understood that the terminology usedherein is for the purpose of describing particular aspects only and isnot intended to be limiting. Also, throughout this specification,various publications are referenced. The disclosures of thesepublications in their entireties are hereby incorporated by referenceinto this application in order to more fully describe the state of theart to which the disclosed matter pertains. The references disclosed arealso individually and specifically incorporated by reference herein forthe material contained in them that is discussed in the sentence inwhich the reference is relied upon.

Throughout the description and claims of this specification the word“comprise” and other forms of the word, such as “comprising” and“comprises,” means including but not limited to, and is not intended toexclude, for example, other additives, components, integers, or steps.

As used in the description and the appended claims, the singular forms“a,” “an,” and “the” include plural referents unless the context clearlydictates otherwise. Thus, for example, reference to “a composition”includes mixtures of two or more such compositions, reference to “anagent” includes mixtures of two or more such agents, reference to “thelayer” includes combinations of two or more such layers, and the like.

“Optional” or “optionally” means that the subsequently described eventor circumstance can or cannot occur, and that the description includesinstances where the event or circumstance occurs and instances where itdoes not.

Certain materials, compounds, compositions, and components disclosedherein can be obtained commercially or readily synthesized usingtechniques generally known to those of skill in the art. For example,the starting materials and reagents used in preparing the disclosedcompounds and compositions are either available from commercialsuppliers or prepared by methods known to those skilled in the art.

Also, disclosed herein are materials, compounds, compositions, andcomponents that can be used for, can be used in conjunction with, can beused in preparation for, or are products of the disclosed methods andcompositions. These and other materials are disclosed herein, and it isunderstood that when combinations, subsets, interactions, groups, etc.of these materials are disclosed that while specific reference of eachvarious individual and collective combinations and permutation of thesecompounds may not be explicitly disclosed, each is specificallycontemplated and described herein. For example, if a composition isdisclosed and a number of modifications that can be made to a number ofcomponents of the composition are discussed, each and every combinationand permutation that are possible are specifically contemplated unlessspecifically indicated to the contrary. Thus, if a class of componentsA, B, and C are disclosed as well as a class of components D, E, and Fand an example of a composition A-D is disclosed, then even if each isnot individually recited, each is individually and collectivelycontemplated. Thus, in this example, each of the combinations A-E, A-F,B-D, B-E, B-F, C-D, C-E, and C-F are specifically contemplated andshould be considered disclosed from disclosure of A, B, and C; D, E, andF; and the example combination A-D. Likewise, any subset or combinationof these is also specifically contemplated and disclosed. Thus, forexample, the sub-group of A-E, B-F, and C-E are specificallycontemplated and should be considered disclosed from disclosure of A, B,and C; D, E, and F; and the example combination A-D. This conceptapplies to all aspects of this disclosure including, but not limited to,steps in methods of making and using the disclosed compositions. Thus,if there are a variety of additional steps that can be performed it isunderstood that each of these additional steps can be performed with anyspecific aspect or combination of aspects of the disclosed methods, andthat each such combination is specifically contemplated and should beconsidered disclosed.

As a particular example of the subset concept, the range for any of thecomponents of the glasses of the invention (including the range for asum of the components) or the range for any property of those glasses,including, in particular, the range for the component or property setforth in a claim, can be narrowed (amended) at either the range's upperor lower end to any value for that component or property disclosedherein, whether the disclosure is a corresponding upper or lower end ofanother range for the component or property (e.g., a preferred range) orthe amount of the component used in a particular example or a propertyexhibited by a particular example. With regard to narrowing claimedranges based on examples, such narrowing applies irrespective of whetheror not the remainder of the example is within the scope of the claimbeing narrowed.

Reference will now be made in detail to specific aspects of thedisclosed materials, compounds, compositions, articles, and methods,examples of which are illustrated in the accompanying Examples andFigures.

Described herein are alkali-free glasses and methods for making the samethat possess high strain points and, thus, good dimensional stability(i.e., low compaction). A high strain point glass can prevent paneldistortion due to compaction/shrinkage during thermal processingsubsequent to manufacturing of the glass.

It has been discovered that a glass with a strain point greater than700° C. will minimize the dimensional changes (i.e., compaction)experienced by glass that is quickly cooled then reheated for a briefperiod of time. In one aspect, the glass compositions described hereinhave a strain point greater than or equal to 700° C. or greater than orequal to 710° C. In a further aspect, the glass compositions describedherein have a strain point between 700° C. and 800° C., 700° C. and 775°C., 700° C. and 750° C., or 700° C. and 730° C. In a further aspect, thehigh strain point glass compositions described herein have a thermalcompaction when subjected to a 600° C. heat treatment for 5 minutes thatis less than 30 ppm, less than 25 ppm, less than 20 ppm, less than 15ppm, or less than 10 ppm, with the lower compactions being preferred.

In one aspect, the glasses described herein have a strain point inexcess of about 700° C. and experience less than 30 ppm compaction(i.e., dimensional change) after heat treatment for five minutes at 600°C. This temperature and duration were chosen to approximate a lowtemperature poly-silicon heat treatment cycle. FIG. 1 shows experimentaldata for a series of glasses with measured strain points along thex-axis, versus measured dimensional change after five minutes at 600° C.

In one aspect, described herein is an alkali-free glass comprising inmole percent on an oxide basis:

SiO₂ 64.0-72.0 Al₂O₃  9.0-16.0 B₂O₃ 1.0-5.0 MgO + La₂O₃ 1.0-7.5 CaO2.0-7.5 SrO 0.0-4.5 BaO 1.0-7.0

wherein Σ(MgO+CaO+SrO+BaO+3La₂O₃)/(Al₂O₃)≥1.15, where Al₂O₃, MgO, CaO,SrO, BaO, and La₂O₃ represent the mole percents of the respective oxidecomponents.

In a further aspect, described herein is an alkali-free glass comprisingin mole percent on an oxide basis:

SiO₂ 64.0-72.0 B₂O₃ 1.0-5.0 Al₂O₃  9.0-16.0 MgO + La₂O₃ 1.0-7.5 CaO2.0-7.5 SrO 0.0-4.5 BaO 1.0-7.0

wherein:

-   -   1.15 Σ≤(MgO+CaO+SrO+BaO+3La₂O₃)/(Al₂O₃)≤1.55, where Al₂O₃, MgO,        CaO, SrO, BaO, and La₂O₃ represent the mole percents of the        respective oxide components;    -   the glass has a strain point greater than or equal to 700° C.;    -   the glass has a temperature at 200 poise viscosity less than or        equal to 1,665° C.; and    -   the glass has a viscosity at the liquidus temperature greater        than or equal to 85,000 poise.

In another aspect, described herein is an alkali-free glass comprisingin mole percent on an oxide basis:

SiO₂ 65.0-71.0 Al₂O₃  9.0-16.0 B₂O₃ 1.5-4.0 MgO + La₂O₃ 0.5-7.5 CaO2.0-6.0 SrO 0.0-4.5 BaO 1.0-7.0 La₂O₃ >0.0 and less than or equal to 4.0

wherein Σ(MgO+CaO+SrO+BaO+3La₂O₃)/(Al₂O₃)≥1.15 (preferably, ≥1.2), whereAl₂O₃, MgO, CaO, SrO, BaO, and La₂O₃ represent the mole percents of therespective oxide components.

In a further aspect, described herein is an alkali-free glass comprisingin mole percent on an oxide basis:

SiO₂ 65.0-72.0 Al₂O₃ 10.0-15.0 B₂O₃ 1.0-4.0 MgO 2.0-7.5 CaO 3.0-6.0 SrO0.0-4.5 BaO 1.0-6.0

wherein Σ(MgO+CaO+SrO+BaO)/(Al₂O₃)≥1.15, where Al₂O₃, MgO, CaO, SrO, andBaO represent the mole percents of the respective oxide components.

In an additional aspect, described herein is an alkali-free glasscomprising SiO₂, Al₂O₃, B₂O₃, MgO, CaO, and at least one of SrO and BaO,wherein the glass' composition satisfies the relationship:[MgO]:[CaO]:[SrO+BaO]=1±0.15:1±0.15:1±0.15 (preferably,[MgO]:[CaO]:[SrO+BaO]=1±0.1:1±0.1:1±0.1),

where [MgO], [CaO], and [SrO+BaO] are the concentrations of theindicated components of the glass in mole percent on an oxide basis.Preferably, in connection with this aspect, the B₂O₃ concentration isless than or equal to 4.0 mole percent. As illustrated in the examples,it has been found that glasses that satisfy the above relationships havelow liquidus temperatures (high liquidus viscosities) as is desired fordowndraw processes, such as the fusion process. As discussed below, theSrO and BaO concentrations can be adjusted to optimize strain point,CTE, and density. Preferably, the BaO/SrO ratio in mole percent is equalto or greater than 2.0.

In the glass compositions described herein, SiO₂ serves as the basicglass former. In certain aspects, the concentration of SiO₂ can begreater than 64 mole percent in order to provide the glass with adensity and chemical durability suitable for a flat panel display glass(e.g., an AMLCD glass), and a liquidus temperature (liquidus viscosity),which allows the glass to be formed by a downdraw process (e.g., afusion process). In terms of an upper limit, in general, the SiO₂concentration can be less than or equal to about 72.0 mole percent toallow batch materials to be melted using conventional, high volume,melting techniques, e.g., Joule melting in a refractory melter. As theconcentration of SiO₂ increases, the 200 poise temperature (meltingtemperature) generally rises. In various applications, the SiO₂concentration is adjusted so that the glass composition has a meltingtemperature less than or equal to 1,650° C. In one aspect, the SiO₂concentration is between 66.0 and 71.0 mole percent or between 66.5 and70.5 mole percent.

Al₂O₃ is another glass former used to make the glasses described herein.An Al₂O₃ concentration greater than or equal to 9.0 mole percentprovides the glass with a low liquidus temperature and a correspondinghigh liquidus viscosity. The use of at least 9.0 mole percent Al₂O₃ alsoimproves the glass' strain point and modulus. In order to achieve anΣ(MgO+CaO+SrO+BaO+3La₂O₃)/(Al₂O₃) ratio greater than or equal to 1.15(see below), it is desirable to keep the Al₂O₃ concentration below 16.0mole percent. In one aspect, the Al₂O₃ concentration is between 12.0 and15.0 mole percent.

B₂O₃ is both a glass former and a flux that aids melting and lowers themelting temperature. To achieve these effects, the glass compositionsdescribed herein have B₂O₃ concentrations that are equal to or greaterthan 1.0 mole percent.

During the processing of displays, glass sheets are often held only atopposite edges, and therefore experience sagging in the unsupportedcentral portion of the sheet. The amount of sag is a function of thegeometry of the sheet, the density, and Young's modulus of the glass.The sheet geometry is dictated by the particular process employed, whichis beyond the control of the glass manufacturer. For fixed density, anincrease in Young's modulus is desirable since it reduces the amount ofsag exhibited by large glass sheets during shipping, handling andthermal processing. Likewise, any increase in density should beaccompanied by a proportionate increase in Young's modulus or it willresult in increased sag. In one aspect, the glass has a density of lessthan or equal to 2.75 grams/cm³. In another aspect, to minimize thecontribution of the glass itself to sag, it is desirable that the ratioof Young's modulus/density is greater than 30.0 GPa·cm³/g, greater than30.5 GPa·cm³/g, greater than 31.2 GPa·cm³/g, or greater than 32.2GPa·cm³/g, with higher ratios being preferred. While B₂O₃ reducesdensity when it replaces alkaline earth oxides, La₂O₃ or Al₂O₃, Young'smodulus decreases more rapidly. Thus, it is desirable to keep B₂O₃content as low as reasonably possible. B₂O₃ also tends to decreasestrain point and, for this reason also, the B₂O₃ content of the glass ispreferably kept as low as possible.

As discussed above with regard to SiO₂, glass durability is also veryimportant for LCD applications. Durability can be controlled somewhat byelevated concentrations of alkaline earths and lanthanum oxides, andsignificantly reduced by elevated B₂O₃ content. As with strain point andYoung's modulus, it is desirable to keep B₂O₃ content low. Thus, toachieve the above properties, in one aspect, the glasses describedherein have B₂O₃ concentrations that are less than or equal to 5.0 molepercent, between 1.0 and 5.0 mole percent, between 1.0 and 4.0 molepercent, or between 2.0 and 4.0 mole percent.

The Al₂O₃ and B₂O₃ concentrations can be selected as a pair to increasestrain point, increase modulus, improve durability, reduce density, andreduce the coefficient of thermal expansion (CTE), while maintaining themelting and forming properties of the glass.

For example, an increase in B₂O₃ and a corresponding decrease in Al₂O₃can be helpful in obtaining a lower density and CTE, while an increasein Al₂O₃ and a corresponding decrease in B₂O₃ can be helpful inincreasing strain point, modulus, and durability, provided that theincrease in Al₂O₃ does not reduce the Σ(MgO+CaO+SrO+BaO)/(Al₂O₃) orΣ(MgO+CaO+SrO+BaO+3La₂O₃)/(Al₂O₃) ratio below about 1.15. For example,as known in the art, glasses for use in AMLCD applications have CTE's(0-300° C.) in the range of 28-42×10⁻⁷/° C., preferably, 30-40×10⁻⁷/°C., and most preferably, 32-38×10⁻⁷/° C. CTE's measured for othertemperature ranges, e.g., 25-300° C. as in the examples presented below,can be transformed to the 0-300° C. range by adding an offset to themeasured value. In the case of transforming CTE values measured for the25-300° C. range to values for the 0-300° C. range, an offset of−0.8×10⁻⁷/° C. has been found to work successfully for AMLCDsilica-based glasses. With regard to CTE, historically, lamp glasseshave had low boron and high alkaline earth contents (leading to highalkaline earth to alumina ratios), but these glasses have purposely hadCTE's above 42×10⁻⁷/° C. so that they would be compatible withmolybdenum lead wires. Accordingly, the lamp glasses are not suitablefor use in AMLCD applications.

In addition to the glass formers (SiO₂, Al₂O₃, and B₂O₃), the glassesdescribed herein also include alkaline earth oxides. In one aspect, atleast three alkaline earth oxides are part of the glass composition,e.g., MgO, CaO, and BaO, and, optionally, SrO. The alkaline earth oxidesprovide the glass with various properties important to melting, fining,forming, and ultimate use. Accordingly, to improve glass performance inthese regards, in one aspect, the Σ(MgO+CaO+SrO+BaO)/(Al₂O₃) ratio isgreater than or equal to 1.15, greater than or equal to 1.2, or greaterthan or equal to 1.25. In another aspect, the Σ(MgO+CaO+SrO+BaO)/(Al₂O₃)ratio is less than or equal to 1.55 or less than or equal to 1.50.

The concentrations of MgO, La₂O₃, or combinations thereof, in the glassand the Σ(MgO+CaO+SrO+BaO+3La₂O₃)/(Al₂O₃) ratio of the glass, caninfluence glass performance, particularly meltability and fining.Accordingly, to improve glass performance in these regards, in oneaspect, the Σ(MgO+CaO+SrO+BaO+3La₂O₃)/(Al₂O₃) ratio is greater than orequal to 1.15, greater than or equal to 1.20, or greater than or equalto 1.25. In another aspect, the Σ(MgO+CaO+SrO+BaO+3La₂O₃)/(Al₂O₃) ratiois greater than or equal to 1.15 and less than or equal to 1.55, orgreater than or equal to 1.25 and less than or equal to 1.45.

For certain embodiments of this invention, MgO and La₂O₃ are treated aswhat is in effect a single compositional component. This is becausetheir impact upon viscoelastic properties, liquidus temperatures andliquidus phase relationships are qualitatively very similar. The otheralkaline earth oxides form feldspar minerals, notably anorthite(CaAl₂Si₂O₈) and celsian (BaAl₂Si₂O₈) and strontium-bearing solidsolutions of same. Neither MgO nor La₂O₃ participate in these crystalsto a significant degree. Therefore, when a feldspar crystal is alreadythe liquidus phase, a superaddition of MgO or La₂O₃ actual serves tostabilize the liquid relative to the crystal and thus lower the liquidustemperature. At the same time, the viscosity curve typically becomessteeper, reducing melting temperatures while having little or no impacton low-temperature viscosities. In this sense, the addition of smallamounts of MgO and/or La₂O₃ benefits melting by reducing meltingtemperatures, benefits forming by reducing liquidus temperatures andincreasing liquidus viscosity, while preserving high strain point and,thus, low compaction.

The impact of MgO and La₂O₃ on these properties is similar, but theirimpact on other key glass properties is very different. La₂O₃ greatlyincreases density and mildly increases CTE relative to MgO. WhileYoung's modulus typically increases when either is added to analuminosilicate glass of the invention, the density increases so quicklywith La₂O₃ content that the specific modulus (Young's modulus divided bydensity) decreases. It is desirable to have a specific modulus of atleast 28 MPa·m³/kg, a typical value for AMLCD substrates in order toavoid excessive sagging of the sheet during thermal processing. Young'smodulus for the glasses of this invention range from about 77.6 to about83 GPa, and thus when density is greater than about 2.75 g/cc thespecific modulus will fall below the desired level. For this reason, itis necessary to constrain La₂O₃ to be no higher than necessary toproduce the desired impact on viscoelastic properties. While MgO doesnot have this impact, at high concentrations it finds high solubility inthe barium aluminosilicate celsian, and thus for liquidus purposes mustbe kept at or below a level comparable to that for La₂O₃.

By increasing the sum of MgO+La₂O₃, the liquidus temperature can riseand the liquidus viscosity can fall to a level such that the use of ahigh viscosity forming process (e.g., a fusion process) is compromised.Therefore, the amount of MgO and La₂O₃ can be adjusted accordingly toobtain the desired properties for glass formation. In terms ofconcentrations, when both are present, the combined concentration ofMgO+La₂O₃ should be between 1.0 and 7.5 mole percent in order to achievethe various benefits described above. In another aspect, the MgOconcentration is between 2.0 and 6.0 mole percent or between 3.0 and 6.0mole percent, especially when MgO is used in the absence of La₂O₃. In afurther aspect, the La₂O₃ concentration is preferably kept less than orequal to about 3.0 mole percent so as not to elevate the density of theglass.

Calcium oxide present in the glass composition can produce low liquidustemperatures (high liquidus viscosities), high strain points and moduli,and CTE's in the most desired ranges for flat panel applications,specifically, AMLCD applications. It also contributes favorably tochemical durability, and compared to other alkaline earth oxides, it isrelatively inexpensive as a batch material. However, at highconcentrations, CaO increases the density and CTE. Furthermore, atsufficiently low SiO₂ concentrations, CaO may stabilize anorthite, thusdecreasing liquidus viscosity. Accordingly, in one aspect, the CaOconcentration can be greater than or equal to 2.0 mole percent. Inanother aspect, the CaO concentration of the glass composition is lessthan or equal to 7.5 mole percent or is between 3.0 and 7.5 molepercent.

The remaining alkaline earth oxides SrO and BaO can both contribute tolow liquidus temperatures (high liquidus viscosities) and, thus, theglasses described herein will typically contain at least one of theseoxides. However, the selection and concentration of these oxides areselected in order to avoid an increase in CTE and density and a decreasein modulus and strain point. The relative proportions of SrO and BaO canbe balanced so as to obtain a suitable combination of physicalproperties and liquidus viscosity such that the glass can be formed by adowndraw process.

To summarize the effects/roles of the central components of the glassesof the invention, SiO₂ is the basic glass former. Al₂O₃ and B₂O₃ arealso glass formers and can be selected as a pair with, for example, anincrease in B₂O₃ and a corresponding decrease in Al₂O₃ being used toobtain a lower density and CTE, while an increase in Al₂O₃ and acorresponding decrease in B₂O₃ being used in increasing strain point,modulus, and durability, provided that the increase in Al₂O₃ does notreduce the RO/(Al₂O₃) or (RO+3La₂O₃)/(Al₂O₃) ratio below about 1.15,where RO=Σ(MgO+CaO+SrO+BaO). If the ratio goes too low, meltability iscompromised, i.e., the melting temperature becomes too high. B₂O₃ can beused to bring the melting temperature down, but high levels of B₂O₃compromise strain point.

In addition to meltability and strain point considerations, for AMLCDapplications, the CTE of the glass should be compatible with that ofsilicon. To achieve such CTE values, the glasses of the inventioncontrol the RO content of the glass (or the RO+3La₂O₃ content forglasses that include La₂O₃). For a given Al₂O₃ content, controlling theRO content corresponds to controlling the RO/Al₂O₃ ratio (or the(RO+3La₂O₃)/Al₂O₃ ratio for glasses that include La₂O₃). In practice,glasses having suitable CTE's are produced if the RO/Al₂O₃ ratio (or the(RO+3La₂O₃)/Al₂O₃ ratio) is below about 1.55.

On top of these considerations, the glasses are preferably formable by adowndraw process, e.g., a fusion process, which means that the glass'liquidus viscosity needs to be relatively high. Individual alkalineearths play an important role in this regard since they can destabilizethe crystalline phases that would otherwise form. BaO and SrO areparticularly effective in controlling the liquidus viscosity and areincluded in the glasses of the invention for at least this purpose. Asillustrated in the examples presented below, various combinations of thealkaline earths will produce glasses having high liquidus viscosities,with the total of the alkaline earths (and La₂O₃ when used) satisfyingthe RO/Al₂O₃ ratio (or the (RO+3La₂O₃)/Al₂O₃ ratio) constraints neededto achieve low melting temperatures, high strain points, and suitableCTE's.

In addition to the above components, the glass compositions describedherein can include various other oxides to adjust various physical,melting, fining, and forming attributes of the glasses. Examples of suchother oxides include, but are not limited to, TiO₂, MnO, Fe₂O₃, ZnO,Nb₂O₅, MoO₃, Ta₂O₅, WO₃, Y₂O₃, and CeO₂. In one aspect, the amount ofeach of these oxides can be less than or equal to 2.0 mole percent, andtheir total combined concentration can be less than or equal to 4.0 molepercent. The glass compositions described herein can also includevarious contaminants associated with batch materials and/or introducedinto the glass by the melting, fining, and/or forming equipment used toproduce the glass, particularly Fe₂O₃ and ZrO₂. The glasses can alsocontain SnO₂ either as a result of Joule melting using tin-oxideelectrodes and/or through the batching of tin containing materials,e.g., SnO₂, SnO, SnCO₃, SnC₂O₄, etc.

The glass compositions are generally alkali free; however, the glassescan contain some alkali contaminants. In the case of AMLCD applications,it is desirable to keep the alkali levels below 0.1 mole percent toavoid having a negative impact on thin film transistor (TFT) performancethrough diffusion of alkali ions from the glass into the silicon of theTFT. As used herein, an “alkali-free glass” is a glass having a totalalkali concentration which is less than or equal to 0.1 mole percent,where the total alkali concentration is the sum of the Na₂O, K₂O, andLi₂O concentrations. In one aspect, the total alkali concentration isless than or equal to 0.07 mole percent.

As discussed above, Σ(MgO+CaO+SrO+BaO)/(Al₂O₃) andΣ(MgO+CaO+SrO+BaO+3La₂O₃)/(Al₂O₃) ratios greater than or equal to 1.15improve fining, i.e., the removal of gaseous inclusions from the meltedbatch materials. This improvement allows for the use of moreenvironmentally friendly fining packages. For example, on an oxidebasis, the glass compositions described herein can have one or more orall of the following compositional characteristics:

an As₂O₃ concentration of at most 0.05 mole percent;

an Sb₂O₃ concentration of at most 0.05 mole percent;

a SnO₂ concentration of at most 0.2 mole percent.

As₂O₃ is an effective high temperature fining agent for AMLCD glasses,and in some aspects described herein, As₂O₃ is used for fining becauseof its superior fining properties. However, As₂O₃ is poisonous andrequires special handling during the glass manufacturing process.Accordingly, in certain aspects, fining is performed without the use ofsubstantial amounts of As₂O₃, i.e., the finished glass has at most 0.05mole percent As₂O₃. In one aspect, no As₂O₃ is purposely used in thefining of the glass. In such cases, the finished glass will typicallyhave at most 0.005 mole percent As₂O₃ as a result of contaminantspresent in the batch materials and/or the equipment used to melt thebatch materials.

Although not as toxic as As₂O₃, Sb₂O₃ is also poisonous and requiresspecial handling. In addition, Sb₂O₃ raises the density, raises the CTE,and lowers the strain point in comparison to glasses that use As₂O₃ orSnO₂ as a fining agent. Accordingly, in certain aspects, fining isperformed without the use of substantial amounts of Sb₂O₃, i.e., thefinished glass has at most 0.05 mole percent Sb₂O₃. In another aspect,no Sb₂O₃ is purposely used in the fining of the glass. In such cases,the finished glass will typically have at most 0.005 mole percent Sb₂O₃as a result of contaminants present in the batch materials and/or theequipment used to melt the batch materials.

Compared to As₂O₃ and Sb₂O₃ fining, tin fining (i.e., SnO₂ fining) isless effective, but SnO₂ is a ubiquitous material that has no knownhazardous properties. Also, for many years, SnO₂ has been a component ofAMLCD glasses through the use of tin oxide electrodes in the Joulemelting of the batch materials for such glasses. The presence of SnO₂ inAMLCD glasses has not resulted in any known adverse effects in the useof these glasses in the manufacture of liquid crystal displays. However,high concentrations of SnO₂ are not preferred as this can result in theformation of crystalline defects in AMLCD glasses. In one aspect, theconcentration of SnO₂ in the finished glass is less than or equal to 0.2mole percent.

Tin fining can be used alone or in combination with other finingtechniques if desired. For example, tin fining can be combined withhalide fining, e.g., bromine fining. Other possible combinationsinclude, but are not limited to, tin fining plus sulfate, sulfide,cerium oxide, mechanical bubbling, and/or vacuum fining. It iscontemplated that these other fining techniques can be used alone. Incertain aspects, maintaining the Σ(MgO+CaO+SrO+BaO+3La₂O₃)/(Al₂O₃) ratioand individual alkaline earth and La₂O₃ concentrations within the rangesdiscussed above makes the fining process easier to perform and moreeffective.

The glasses described herein can be manufactured using varioustechniques known in the art. In one aspect, the glasses are made using adowndraw process such as, for example, a fusion downdraw process. In oneaspect, described herein is a method for producing an alkali-free glasssheet by a downdraw process comprising selecting, melting, and finingbatch materials so that the glass making up the sheets comprises SiO₂,Al₂O₃, B₂O₃, MgO, CaO and BaO, and, on an oxide basis, comprises:

a Σ≤(MgO+CaO+SrO+BaO)/(Al₂O₃) ratio greater than or equal to 1.15;

(ii) a MgO content greater than or equal to 2.0 mole percent;

a CaO content greater than or equal to 3.0 mole percent; and

a BaO content greater than or equal to 1.0 mole percent,

wherein:

the fining is performed without the use of substantial amounts ofarsenic (and, optionally, without the use of substantial amounts ofantimony); and

a population of 50 sequential glass sheets produced by the downdrawprocess from the melted and fined batch materials has an average gaseousinclusion level of less than 0.10 gaseous inclusions/cubic centimeter,where each sheet in the population has a volume of at least 500 cubiccentimeters.

U.S. Pat. No. 5,785,726 (Dorfeld et al.), U.S. Pat. No. 6,128,924 (Bangeet al.), U.S. Pat. No. 5,824,127 (Bange et al.), and co-pending patentapplication Ser. No. 11/116,669 disclose processes for manufacturingarsenic free glasses.

In one aspect, the population of 50 sequential glass sheets produced bythe downdraw process from the melted and fined batch materials has anaverage gaseous inclusion level of less than 0.05 gaseousinclusions/cubic centimeter, where each sheet in the population has avolume of at least 500 cubic centimeters.

The downdraw sheet drawing processes and, in particular, the fusionprocess described in U.S. Pat. Nos. 3,338,696 and 3,682,609 both toDockerty, which are incorporated by reference, can be used herein.Compared to other forming processes, such as the float process, thefusion process is preferred for several reasons. First, glass substratesmade from the fusion process do not require polishing. Current glasssubstrate polishing is capable of producing glass substrates having anaverage surface roughness greater than about 0.5 nm (Ra), as measured byatomic force microscopy. The glass substrates produced by the fusionprocess have an average surface roughness as measured by atomic forcemicroscopy of less than 0.5 nm. The substrates also have an averageinternal stress as measured by optical retardation which is less than orequal to 150 psi.

To be formed by a downdraw process, it is desirable that the glasscompositions described herein have a liquidus viscosity greater than orequal to 85,000 poises, greater than or equal to 100,000 poises, orgreater than or equal to 150,000 poises, higher liquidus viscositiesbeing preferable.

The glass compositions described herein can be used to make variousglass articles. For example, the glass compositions described herein canbe used to make substrates for liquid crystal displays such as, forexample, AMLCDs. In one aspect, to be suitable for use in flat paneldisplay applications such as, for example, AMLCDs, the glasses describedherein have a value for (Young's modulus)/density >30.5 GPa·cm³/g, aweight loss which is less than or equal to 0.8 milligrams/cm² when apolished sample is exposed to a 5% HCl solution for 24 hours at 95° C.,and a weight loss of less than 1.5 milligrams/cm² when exposed to asolution of 1 volume of 50 wt. % HF and 10 volumes 40 wt. % NH₄F at 30°C. for 5 minutes.

EXAMPLES

The following examples are set forth below to illustrate the methods andresults according to the disclosed subject matter. These examples arenot intended to be inclusive of all aspects of the subject matterdisclosed herein, but rather to illustrate representative methods andresults. These examples are not intended to exclude equivalents andvariations of the present invention which are apparent to one skilled inthe art.

Efforts have been made to ensure accuracy with respect to numbers (e.g.,amounts, temperature, etc.) but some errors and deviations should beaccounted for. Unless indicated otherwise, temperature is in ° C. or isat ambient temperature, and pressure is at or near atmospheric. Thecompositions themselves are given in mole percent on an oxide basis andhave been normalized to 100%. There are numerous variations andcombinations of reaction conditions, e.g., component concentrations,temperatures, pressures and other reaction ranges and conditions thatcan be used to optimize the product purity and yield obtained from thedescribed process. Only reasonable and routine experimentation will berequired to optimize such process conditions.

Example 1 Preparation of a Test Sample

Test glass samples are made by melting appropriate batch materials in Ptcrucibles at 1,600-1,650° C. for 6 or more hours, and pouring onto asteel sheet, followed by conventional annealing prior to cutting andpolishing for testing. The resulting patty of glass is processed toyield rectangular or square glass samples roughly 3″×7″×2 mm thick or4″×4″×2 mm thick. The glass rectangles or squares are heated to atemperature corresponding to a viscosity of 10¹¹ poise and held for fourhours, followed by quick cooling to room temperature. This heattreatment is believed to give the best possible approximation to thethermal history of fusion drawn sheet. The samples are then polished ontheir flat surfaces, marked with sets of several fiducial marks neartheir edges (and perpendicular to the long axis of the sample in thecase of rectangular samples). Rectangular samples are cut in halflengthwise, leaving one reference portion, and another portion thatundergoes heat treatment(s). The reference piece and the heat-treatedpiece are examined together under a microscope, and dimensional changesare recorded. Dimensions of square samples are precisely measured beforeheat treatment using a Mitutoyo Quick Vision QV202 Pro instrument,followed by heat treatment(s) at appropriate times and temperatures, andre-measurement of sample dimensions. Because the automated opticalinstrument makes several tens of repeat measurements on each sample,statistical methods can be used to determine dimensional changes as lowas 1 micron, corresponding to less than a 5 to 10 ppm dimensional changefor the sample sizes used. Repeated heat treatments and measurements canbe used to determine the dimensional relaxation behavior of a glass at agiven temperature as a function of time.

FIG. 1 shows the results of compaction testing performed on a series ofcommercial and laboratory glasses to investigate the effects of strainpoint on compaction. As can be seen in this figure, the change incompaction with strain point is a non-linear phenomenon, with thelargest improvement occurring over the 20° C. range between 660° C. and680° C. In this regard, it should be noted that 660° C. is a typicalstrain point value for AMLCD glasses currently on the market. Byincreasing the strain point by just 20° C. to 680° C., the compactiondrops from above 70 ppm to less than 30 ppm, i.e., an improvement ofmore than 50%. For some poly-Si processes, this improvement may besufficient. For other processes, further improvements may be required.As illustrated in FIG. 1, increases in the strain point from 660° C. by50-100° C. or more, achieve compaction levels of 20 ppm and below. Theselevels are suitable for poly-Si processes currently in use or expectedto be used in the coming years.

FIG. 2 shows the effects of repeated heat treatments on glasses havingdifferent strain points. Repeated heat treatments more accurately mimicthe poly-Si manufacturing process than the single heat treatment used togenerate the data of FIG. 1. For FIG. 2, each of three glasses, i.e.,Glass A which corresponds to Example 68 of Table 1, Glass B whichcorresponds to Example 65 of Table 1, and Glass C which is Corning Eagle2000F glass, was inserted in a furnace at 600° C. and held attemperature for increasing periods of time as the experiment continued.For the first heat treatment, the period was 5 minutes, for the secondperiod it was an additional five minutes (for a total time of 10minutes), and so forth as shown in Table 2, with the last four periodsbeing for 60 minutes each. After each heat treatment, the glass samplewas removed from the furnace and placed between two fans to achieverapid cooling. In this way, the glass samples were basically subjectedto step changes in their temperatures.

As can be seen in FIG. 2, for all three samples, the compaction per testperiod decreases with increasing time since the glass structure hasrelaxed based on its prior exposures to the 600° C. treatments. However,the glasses with higher strain points, i.e., Glasses A and B of theinvention, exhibited markedly lower levels of compaction than Glass Cwhich had a strain point characteristic of currently available AMLCDglasses. The data plotted in FIG. 2 is set forth in Table 2.

In addition to low compaction, the glass must satisfy rigorous formingrequirements to be applicable to fusion draw or related processes.Devitrification is defined as the formation of a crystalline phase froman initially homogeneous glass. The maximum temperature at whichcrystals coexist with glass is defined as the liquidus temperature. Asdescribed below in connection with the testing of the glasses of Table1, liquidus temperature is measured by loading a crushed glass sampleinto a platinum boat, then heating for 24 hours in a tube furnace with agradient of 10° C. or less per cm. The viscosity of the glass at theliquidus temperature is referred to as the liquidus viscosity. Precisionsheet glass forming processes generally require comparatively highliquidus viscosities, for example, greater than 40,000 poise,preferably, greater than 85,000 poise.

The glass must also satisfy rigorous melting requirements for productionpurposes. The temperature at which glass batch constituents melt in aeconomically reasonable amount of time, and the temperature at whichtrapped bubbles of air can rise out of the glass in a reasonable amountof time, typically corresponds to a viscosity of about 200 poises.Limitations in durable refractory or precious metal containers at hightemperatures place an upper practical limit for 200 poises temperatureof about 1,680° C. It is possible that changes in batch materials fromthose conventionally used would allow more viscous glasses to be meltedat proportionately higher viscosities, but such materials invariably addformidable costs, and transporting glass through a melting andconditioning system at high viscosity presents significant technicalchallenges. Numerous glass compositions with their physical propertiesare presented in Table 1.

The glass properties set forth in Table 1 were determined in accordancewith techniques conventional in the glass art. Thus, the linearcoefficient of thermal expansion (CTE) over the temperature range25-300° C. is expressed in terms of ×10⁻⁷/° C. and the strain point isexpressed in terms of ° C. These were determined from fiber elongationtechniques (ASTM references E228-85 and C336, respectively).

The density in terms of grams/cm³ was measured via the Archimedes method(ASTM C693).

The melting temperature in terms of ° C. (defined as the temperature atwhich the glass melt demonstrates a viscosity of 200 poises) wascalculated employing a Fulcher equation fit to high temperatureviscosity data measured via rotating cylinders viscometry (ASTMC965-81).

The liquidus temperature of the glass in terms of ° C. was measuredusing the standard gradient boat liquidus method of ASTM C829-81. Thisinvolves placing crushed glass particles in a platinum boat, placing theboat in a furnace having a region of gradient temperatures, heating theboat in an appropriate temperature region for 24 hours, and determiningby means of microscopic examination the highest temperature at whichcrystals appear in the interior of the glass. More particularly, theglass sample is removed from the Pt boat in one piece, and examinedusing polarized light microscopy to identify the location and nature ofcrystals which have formed against the Pt and air interfaces, and in theinterior of the sample. Because the gradient of the furnace is very wellknown, temperature vs location can be well estimated, within 5-10° C.The temperature at which crystals are observed in the internal portionof the sample is taken to represent the liquidus of the glass (for thecorresponding test period). Testing is sometimes carried out at longertimes (e.g. 72 hours), in order to observe slower growing phases. Thecrystalline phase for the various glasses of Table 1 are described bythe following abbreviations: anor—anorthite, a calcium aluminosilicatemineral; cris—cristobalite (SiO₂); cels—mixed alkaline earth celsian;Sr/Al sil—a strontium aluminosilicate phase; SrSi—a strontium silicatephase. The liquidus viscosity in poises was determined from the liquidustemperature and the coefficients of the Fulcher equation.

Young's modulus values in terms of Gpa were determined using a resonantultrasonic spectroscopy technique of the general type set forth in ASTME1875-00e1.

As can be seen in Table 1, Examples 1-88 have densities, CTE's, strainpoints, and Young's modulus values that make the glasses suitable foruse in display applications, such as AMLCD applications. Although notshown in Table 1, the glasses also have chemical durabilities suitablefor these applications. In particular, Examples 68, 69, 70, and 71 wereeach found to have weight loss values when immersed in 10 wt. % HF for20 minutes at room temperature of between 6.7 and 7.5 milligrams/cm².For comparison, commercial AMLCD glasses show weight losses in the rangeof 5-8 milligrams/cm² when tested in this manner. The glasses of all theexamples can be formed using downdraw techniques, such as the fusiontechnique. Thus, they have liquidus temperatures less than or equal to1250° C. and liquidus viscosities equal to or greater than 85,000poises, and in most cases, equal to or greater than 150,000 poises.

Glasses having the composition and properties shown in Examples 71 and79 are currently regarded as representing the most preferred embodimentsof the invention, that is, as providing the best combination ofproperties for the purposes of the invention at this time.

Throughout this application, various publications are referenced. Thedisclosures of these publications in their entireties are herebyincorporated by reference into this application in order to more fullydescribe the compounds, compositions and methods described herein.

Various modifications and variations can be made to the materials,methods, and articles described herein. Other aspects of the materials,methods, and articles described herein will be apparent fromconsideration of the specification and practice of the materials,methods, and articles disclosed herein. It is intended that thespecification and examples be considered as exemplary.

TABLE 1 Mol % min max 1 2 3 4 SiO₂ 66.58 70.69 67.63 66.58 68.76 67.61Al₂O₃ 11.75 13.51 12.15 12.14 11.76 11.89 B₂O₃ 1.25 4.96 3.98 3.99 4.963.94 MgO 0.84 7.48 5.28 5.66 3.02 5.42 CaO 2.99 5.89 5.28 5.66 5.57 5.42SrO 0.00 4.29 0.00 1.96 0.68 1.88 BaO 1.30 5.55 5.28 3.77 5.00 3.60La₂O₃ 0.00 3.01 0.00 0.00 0.00 0.00 As₂O₃ 0.00 0.40 0.40 0.25 0.25 0.25Sb₂O₃ 0.00 0.49 0.00 0.00 0.00 0.00 ZrO₂ 0.00 0.02 0.00 0.00 0.00 0.00SnO₂ 0.00 0.08 0.00 0.00 0.00 0.00 MgO + La₂O₃ 2.35 7.48 5.28 5.66 3.025.42 Sum(RO + La2/3O)* 14.27 18.39 15.84 17.05 14.27 16.32 Sum(RO +La2/3O)/Al₂O₃ 1.15 1.41 1.30 1.40 1.21 1.37 Strain Point (° C.) 695 739— 706 715 702 CTE (×10−7/° C.) 35.8 39.6 — 39 37.6 38.5 density 2.5712.842 2.631 2.628 2.610 2.619 Young's modulus (Gpa) 77.6 82.6 — — 78.1 —specific modulus 29.1 31.5 — — 29.9 — 200 poise T (° C.) 1585 1672 —1585 1665 1605 Liquidus T (° C.) 1110 1250 1110 1115 1120 1125 Liquidusphase — — cels cris anor anor Liquidus Viscosity 1.09E+05 1.08E+06 —4.59E+05 1.08E+06 4.48E+05 Mol % 5 6 7 8 9 10 SiO₂ 67.43 68.90 67.9567.20 69.81 69.59 Al₂O₃ 11.75 12.02 12.03 11.89 11.89 11.99 B₂O₃ 3.933.82 3.94 3.99 4.00 3.94 MgO 5.48 4.15 5.23 5.54 2.70 3.05 CaO 5.48 5.785.23 5.54 5.81 5.78 SrO 1.89 1.04 0.00 1.92 0.89 0.93 BaO 3.64 4.29 5.233.68 4.90 4.72 La₂O₃ 0.00 0.00 0.00 0.00 0.00 0.00 As₂O₃ 0.40 0.00 0.390.25 0.00 0.00 Sb₂O₃ 0.00 0.00 0.00 0.00 0.00 0.00 ZrO₂ 0.00 0.00 0.000.00 0.00 0.00 SnO₂ 0.00 0.00 0.00 0.00 0.00 0.00 MgO + La₂O₃ 5.48 4.155.23 5.54 2.70 3.05 Sum(RO + La2/3O) 16.49 15.26 15.69 16.68 14.30 14.48Sum(RO + La2/3O)/Al₂O₃ 1.40 1.27 1.30 1.40 1.20 1.21 Strain Point (° C.)703 705 — 706 701 705 CTE (×10−7/° C.) 37.6 37.5 — 39.4 — 37.2 density2.611 2.598 2.628 2.616 2.593 2.593 Young's modulus (Gpa) 80 — — — — —specific modulus 30.6 — — — — — 200 poise T (° C.) 1618 1642 — 1601 —1664 Liquidus T (° C.) 1125 1130 1130 1130 1135 1135 Liquidus phase anoranor cels anor anor + cris anor Liquidus Viscosity 4.78E+05 5.78E+05 —3.43E+05 — 6.64E+05 Mol % 11 12 13 14 15 16 SiO₂ 70.69 69.83 69.76 69.3668.01 67.93 Al₂O₃ 12.48 11.89 11.95 11.96 12.49 12.37 B₂O₃ 4.72 3.974.00 3.90 3.94 3.94 MgO 0.84 2.70 2.78 3.90 5.09 5.16 CaO 4.69 5.80 5.775.70 5.09 5.16 SrO 1.36 0.89 0.89 0.88 1.76 1.79 BaO 3.71 4.92 4.85 4.303.38 3.41 La₂O₃ 1.51 0.00 0.00 0.00 0.00 0.00 As₂O₃ 0.00 0.00 0.00 0.000.25 0.25 Sb₂O₃ 0.00 0.00 0.00 0.00 0.00 0.00 ZrO₂ 0.00 0.00 0.00 0.000.00 0.00 SnO₂ 0.00 0.00 0.00 0.00 0.00 0.00 MgO + La₂O₃ 2.35 2.70 2.783.90 5.09 5.16 Sum(RO + La2/3O) 15.13 14.31 14.29 14.78 15.32 15.52Sum(RO + La2/3O)/Al₂O₃ 1.21 1.20 1.20 1.24 1.23 1.25 Strain Point (° C.)719 703 704 698 708 707 CTE (×10−7/° C.) 36.7 37.1 — 37.3 37 37.6density 2.673 2.592 — 2.595 2.597 2.600 Young's modulus (Gpa) 77.7 77.63— 79.01 — — specific modulus 29.1 29.9 — 30.5 — — 200 poise T (° C.)1648.19 1672 1672 1651 1624 1620 Liquidus T (° C.) 1140 1140 1140 11401140 1140 Liquidus phase cris anor anor anor anor anor LiquidusViscosity 7.01E+05 7.11E+05 6.53E+05 4.81E+05 4.19E+05 3.78E+05 Mol % 1718 19 20 21 22 SiO₂ 67.85 68.26 66.89 70.25 67.69 69.80 Al₂O₃ 12.2511.91 12.01 12.99 12.01 13.51 B₂O₃ 3.94 3.91 3.99 4.52 3.94 4.32 MgO5.22 5.18 5.60 0.88 5.36 0.90 CaO 5.22 5.18 5.60 4.37 5.36 4.06 SrO 1.810.00 1.94 1.26 1.85 1.16 BaO 3.47 5.18 3.73 3.47 3.55 3.24 La₂O₃ 0.000.00 0.00 2.26 0.00 3.01 As₂O₃ 0.25 0.39 0.25 0.00 0.25 0.00 Sb₂O₃ 0.000.00 0.00 0.00 0.00 0.00 ZrO₂ 0.00 0.00 0.00 0.00 0.00 0.00 SnO₂ 0.000.00 0.00 0.00 0.00 0.00 MgO + La₂O₃ 5.22 5.18 5.60 3.14 5.36 3.91Sum(RO + La2/3O) 15.72 15.54 16.87 16.76 16.12 18.39 Sum(RO +La2/3O)/Al₂O₃ 1.28 1.30 1.40 1.29 1.34 1.36 Strain Point (° C.) 703 —703 725 705 729 CTE (×10−7/° C.) 37.8 — 39.6 37.2 38.7 39.3 Density2.605 2.622 2.623 2.723 2.598 2.842 Young's modulus (Gpa) — — — 79.57 —82.6 specific modulus — — — 29.2 — 29.1 200 poise T (° C.) 1609 — 16061620 1600 1586 Liquidus T (° C.) 1140 1140 1140 1145 1145 1150 Liquidusphase anor cels anor cris cris cris Liquidus Viscosity 3.63E+05 —2.78E+05 4.78E+05 2.56E+05 3.02E+05 Mol % 23 24 25 26 27 28 SiO₂ 68.2967.77 68.28 68.38 68.38 68.20 Al₂O₃ 12.11 12.13 12.07 12.04 12.04 11.89B₂O₃ 3.68 3.94 2.69 2.99 2.99 2.99 MgO 4.46 5.29 5.30 5.45 5.45 5.54 CaO5.82 5.29 5.78 5.45 5.45 5.54 SrO 1.44 1.83 1.92 1.09 0.00 1.92 BaO 4.203.51 3.64 4.36 5.45 3.68 La₂O₃ 0.00 0.00 0.00 0.00 0.00 0.00 As₂O₃ 0.000.25 0.26 0.25 0.25 0.25 Sb₂O₃ 0.00 0.00 0.00 0.00 0.00 0.00 ZrO₂ 0.000.00 0.02 0.00 0.00 0.00 SnO₂ 0.00 0.00 0.03 0.00 0.00 0.00 MgO + La₂O₃4.46 5.29 5.30 5.45 5.45 5.54 Sum(RO + La2/3O) 15.92 15.92 16.64 16.3516.35 16.68 Sum(RO + La2/3O)/Al₂O₃ 1.31 1.31 1.38 1.36 1.36 1.40 StrainPoint (° C.) 698 705 — — — 715 CTE (×10−7/° C.) 37.9 37.3 — — — 39.2Density 2.602 2.607 — 2.627 2.655 2.631 Young's modulus (Gpa) 79.98 — —— — — specific modulus 30.7 — — — — — 200 poise T (° C.) 1635 1611 1611— — 1614 Liquidus T (° C.) 1150 1150 1150 1150 1150 1150 Liquidus phaseanor anor cris cris + cels cels anor Liquidus Viscosity 3.39E+052.56E+05 2.27E+05 — — 2.91E+05 Mol % 29 30 31 32 33 34 SiO₂ 68.38 68.3867.74 68.38 69.53 68.53 Al₂O₃ 12.04 12.04 12.08 12.04 11.95 12.15 B₂O₃2.99 2.99 2.99 2.99 3.93 3.69 MgO 5.45 5.45 5.63 5.68 3.12 5.08 CaO 5.455.45 5.63 4.98 5.46 5.08 SrO 1.56 1.82 1.95 2.13 2.37 0.00 BaO 3.89 3.633.74 3.55 3.39 5.08 La₂O₃ 0.00 0.00 0.00 0.00 0.00 0.00 As₂O₃ 0.25 0.250.25 0.25 0.25 0.40 Sb₂O₃ 0.00 0.00 0.00 0.00 0.00 0.00 ZrO₂ 0.00 0.000.00 0.00 0.00 0.00 SnO₂ 0.00 0.00 0.00 0.00 0.00 0.00 MgO + La₂O₃ 5.455.45 5.63 5.68 3.12 5.08 Sum(RO + La2/3O) 16.35 16.35 16.95 16.34 14.3415.24 Sum(RO + La2/3O)/Al₂O₃ 1.36 1.36 1.40 1.36 1.20 1.25 Strain Point(° C.) — — 715 — 717 703 CTE (×10−7/° C.) — — 39.2 — 38 38 density 2.6202.628 2.636 2.617 2.595 2.620 Young's modulus (Gpa) — — — — — — specificmodulus — — — — — — 200 poise T (° C.) — — 1609 — 1664 — Liquidus T (°C.) 1155 1155 1155 1155 1160 1160 Liquidus phase cels anor anor anoranor cels Liquidus Viscosity — — 2.37E+05 — 3.68E+05 — Mol % 35 36 37 3839 40 SiO₂ 68.70 68.38 67.53 67.53 67.86 68.38 Al₂O₃ 12.20 12.04 11.7711.77 12.23 12.04 B₂O₃ 2.89 2.99 3.89 3.89 2.73 2.99 MgO 5.16 5.45 5.495.99 5.98 6.02 CaO 5.83 5.45 5.49 4.49 5.24 4.29 SrO 1.46 1.82 3.09 1.901.95 2.58 BaO 3.76 3.63 2.49 4.19 3.69 3.44 La₂O₃ 0.00 0.00 0.00 0.000.00 0.00 As₂O₃ 0.00 0.25 0.25 0.25 0.26 0.25 Sb₂O₃ 0.00 0.00 0.00 0.000.00 0.00 ZrO₂ 0.00 0.00 0.00 0.00 0.02 0.00 SnO₂ 0.00 0.00 0.00 0.000.03 0.00 MgO + La₂O₃ 5.16 5.45 5.49 5.99 5.98 6.02 Sum(RO + La2/3O)16.21 16.35 16.56 16.57 16.86 16.33 Sum(RO + La2/3O)/Al₂O₃ 1.33 1.361.41 1.41 1.38 1.36 Strain Point (° C.) 712 — 707 — — — CTE (×10−7/° C.)37.9 — 38.7 — — — density 2.607 2.619 2.608 2.630 2.618 2.624 Young'smodulus (Gpa) 81.15 — — — — — specific modulus 31.1 — — — — — 200 poiseT (° C.) 1629 — 1601 — — — Liquidus T (° C.) 1160 1160 1160 1160 11601160 Liquidus phase anor cris anor cels anor cris + anor LiquidusViscosity 2.68E+05 — 1.66E+05 — — — Mol % 41 42 43 44 45 46 SiO₂ 67.5368.38 67.73 67.27 68.38 68.34 Al₂O₃ 11.77 12.04 12.23 12.27 12.04 12.22B₂O₃ 3.89 2.99 3.19 2.99 2.99 2.60 MgO 6.48 4.67 5.27 5.72 4.99 5.37 CaO5.49 5.84 5.89 5.72 5.68 5.85 SrO 0.90 1.17 2.06 1.98 2.12 1.95 BaO 3.694.67 3.63 3.81 3.55 3.68 La₂O₃ 0.00 0.00 0.00 0.00 0.00 0.00 As₂O₃ 0.250.25 0.00 0.25 0.25 0.00 Sb₂O₃ 0.00 0.00 0.00 0.00 0.00 0.00 ZrO₂ 0.000.00 0.00 0.00 0.00 0.00 SnO₂ 0.00 0.00 0.00 0.00 0.00 0.00 MgO + La₂O₃6.48 4.67 5.27 5.72 4.99 5.37 Sum(RO + La2/3O) 16.56 16.35 16.85 17.2316.34 16.85 Sum(RO + La2/3O)/Al₂O₃ 1.41 1.36 1.38 1.40 1.36 1.38 StrainPoint (° C.) — — 703 719 — 715 CTE (×10−7/° C.) — — 37.9 38.6 — 37.9density 2.600 2.642 2.601 2.640 2.623 2.609 Young's modulus (Gpa) — —81.63 — — 82.12 specific modulus — — 31.4 — — 31.5 200 poise T (° C.) —— 1618 1603 — 1626 Liquidus T (° C.) 1160 1165 1165 1165 1170 1170Liquidus phase cels anor anor anor cris anor Liquidus Viscosity — —2.00E+05 1.70E+05 — 2.12E+05 Mol % 47 48 49 50 51 52 SiO₂ 67.53 67.5367.53 68.38 67.97 68.38 Al₂O₃ 11.77 11.77 11.77 12.04 12.23 12.04 B₂O₃3.89 3.89 3.89 2.99 2.90 2.99 MgO 5.49 6.48 6.48 5.84 5.30 6.52 CaO 5.494.49 5.49 4.67 5.89 3.27 SrO 4.29 1.90 0.00 1.17 2.04 3.27 BaO 1.30 3.694.59 4.67 3.67 3.27 La₂O₃ 0.00 0.00 0.00 0.00 0.00 0.00 As₂O₃ 0.25 0.250.25 0.25 0.00 0.25 Sb₂O₃ 0.00 0.00 0.00 0.00 0.00 0.00 ZrO₂ 0.00 0.000.00 0.00 0.00 0.00 SnO₂ 0.00 0.00 0.00 0.00 0.00 0.00 MgO + La₂O₃ 5.496.48 6.48 5.84 5.30 6.52 Sum(RO + La2/3O) 16.57 16.56 16.56 16.35 16.9016.33 Sum(RO + La2/3O)/Al₂O₃ 1.41 1.41 1.41 1.36 1.38 1.36 Strain Point(° C.) 704 — — — 713 — CTE (×10−7/° C.) 38.7 — — — 37.9 — Density 2.5942.621 2.614 2.634 2.606 2.625 Young's modulus (Gpa) — — — — 82.05 —specific modulus — — — — 31.5 — 200 poise T (° C.) 1588 — — — 1628 —Liquidus T (° C.) 1170 1170 1170 1175 1180 1180 Liquidus phase anor celscels cels anor cels Liquidus Viscosity 1.11E+05 — — — 1.66E+05 — Mol %53 54 55 56 57 58 SiO₂ 68.76 68.38 68.38 67.53 67.53 68.38 Al₂O₃ 12.2612.04 12.04 11.77 11.77 12.04 B₂O₃ 2.56 2.99 2.99 3.89 3.89 2.99 MgO5.35 5.71 6.11 7.38 7.48 6.35 CaO 5.80 4.09 4.09 5.49 3.49 3.63 SrO 1.573.27 2.04 0.00 1.90 2.72 BaO 3.70 3.27 4.09 3.69 3.69 3.63 La₂O₃ 0.000.00 0.00 0.00 0.00 0.00 As₂O₃ 0.00 0.25 0.25 0.25 0.25 0.25 Sb₂O₃ 0.000.00 0.00 0.00 0.00 0.00 ZrO₂ 0.00 0.00 0.00 0.00 0.00 0.00 SnO₂ 0.000.00 0.00 0.00 0.00 0.00 MgO + La₂O₃ 5.35 5.71 6.11 7.38 7.48 6.35Sum(RO + La2/3O) 16.42 16.34 16.33 16.56 16.56 16.33 Sum(RO +La2/3O)/Al₂O₃ 1.34 1.36 1.36 1.41 1.41 1.36 Strain Point (° C.) 714 — —— — — CTE (×10−7/° C.) 37.9 — — — — — Density 2.612 2.627 2.625 2.5792.606 2.627 Young's modulus (Gpa) 81.63 — — — — — specific modulus 31.3— — — — — 200 poise T (° C.) 1631 — — — — — Liquidus T (° C.) 1185 11901190 1190 1190 1195 Liquidus phase anor Batch stone with cris cris celscris cris + cels Liquidus Viscosity 1.60E+05 — — — — — Mol % 59 60 61 6263 64 SiO₂ 68.53 69.53 68.38 68.38 67.63 67.63 Al₂O₃ 12.15 11.95 12.0412.04 12.15 12.15 B₂O₃ 3.69 3.93 2.99 2.99 3.98 3.98 MgO 6.27 3.12 4.904.09 6.87 5.28 CaO 2.99 5.46 4.90 5.71 2.99 5.28 SrO 2.99 3.97 3.27 3.272.99 0.00 BaO 2.99 1.80 3.27 3.27 2.99 5.28 La₂O₃ 0.00 0.00 0.00 0.000.00 0.00 As₂O₃ 0.40 0.25 0.25 0.25 0.40 0.40 Sb₂O₃ 0.00 0.00 0.00 0.000.00 0.00 ZrO₂ 0.00 0.00 0.00 0.00 0.00 0.00 SnO₂ 0.00 0.00 0.00 0.000.00 0.00 MgO + La₂O₃ 6.27 3.12 4.90 4.09 6.87 5.28 Sum(RO + La2/3O)15.24 14.35 16.34 16.34 15.84 15.84 Sum(RO + La2/3O)/Al₂O₃ 1.25 1.201.36 1.36 1.30 1.30 Strain Point (° C.) 709 721 — — 702 694.5 CTE(×10−7/° C.) 36.2 36.9 — — 36.2 38.6 density 2.602 2.571 2.640 2.6422.610 2.638 Young's modulus (Gpa) — — — — — — specific modulus — — — — —— 200 poise T (° C.) — 1648 — — — — Liquidus T (° C.) 1200 1205 12151220 1230 1250 Liquidus phase Sr/Al sil anor anor anor SrSi celsLiquidus Viscosity — 1.22E+05 — — — — Mol % 65 66 67 68 69 70 SiO₂ 69.1269.43 69.43 68.61 69.22 69.41 Al₂O₃ 12.01 11.88 11.87 12.67 12.21 12.46B₂O₃ 2.01 2.00 2.00 2.23 2.00 2.00 MgO 5.61 5.55 5.95 5.25 5.42 5.22 CaO5.61 5.55 4.76 5.59 5.42 5.22 SrO 1.59 0.00 1.19 1.50 1.53 1.48 BaO 4.015.55 4.76 3.59 3.73 3.73 La₂O₃ 0.00 0.00 0.00 0.00 0.00 0.00 As₂O₃ 0.000.00 0.00 0.00 0.00 0.00 Sb₂O₃ 0.00 0.00 0.00 0.48 0.40 0.40 ZrO₂ 0.020.02 0.02 0.00 0.00 0.02 SnO₂ 0.02 0.02 0.02 0.08 0.07 0.07 MgO + La₂O₃5.61 5.55 5.95 5.25 5.42 5.22 Sum(RO + La2/3O) 16.82 16.65 16.66 15.9316.10 15.65 Sum(RO + La2/3O)/Al₂O₃ 1.40 1.40 1.40 1.26 1.32 1.26 StrainPoint (° C.) 733 736 736 728 727 730 CTE (×10−7/° C.) 38.9 38.5 38.037.0 37.9 36.8 density 2.630 2.640 2.643 2.637 2.640 2.635 Young'smodulus (Gpa) — — — 81.4 81.4 81.4 specific modulus — — — 30.9 30.8 30.9200 poise T (° C.) 1633 1657 1652 1639 1644 1649 Liquidus T (° C.) 11551160 1155 1185 1170 1180 Liquidus phase cels cels cels cels cels celsLiquidus Viscosity 2.60E+05 3.50E+05 4.29E+05 1.98E+05 2.87E+05 2.63E+05Mol % 71 72 73 74 75 76 SiO₂ 69.38 69.47 69.37 69.77 69.85 69.85 Al₂O₃12.44 12.55 13.07 12.87 12.83 12.88 B₂O₃ 1.99 2.00 2.00 2.00 2.00 2.00MgO 5.21 5.05 5.24 4.93 4.93 4.93 CaO 5.21 5.22 4.81 4.93 4.93 4.93 SrO1.48 1.47 0.75 1.64 1.40 1.62 BaO 3.72 3.68 4.20 3.30 3.50 3.30 La₂O₃0.00 0.00 0.00 0.00 0.00 0.00 As₂O₃ 0.00 0.00 0.00 0.00 0.00 0.00 Sb₂O₃0.48 0.47 0.47 0.47 0.47 0.40 ZrO₂ 0.02 0.02 0.02 0.02 0.02 0.02 SnO₂0.07 0.07 0.07 0.07 0.07 0.07 MgO + La₂O₃ 5.21 5.05 5.24 4.93 4.93 4.93Sum(RO + La2/3O) 15.62 15.42 15.00 14.80 14.76 14.78 Sum(RO +La2/3O)/Al₂O₃ 1.26 1.23 1.15 1.15 1.15 1.15 Strain Point (° C.) 731 729734 734 738 739 CTE (×10−7/° C.) 37.2 37.3 35.8 36.0 36.1 36.0 density2.638 2.635 2.642 2.619 2.620 2.615 Young's modulus (Gpa) 80.7 — — — — —specific modulus 30.6 — — — — — 200 poise T (° C.) 1652 1643 1648 16531652 1659 Liquidus T (° C.) 1170 1180 1220 1195 1195 1200 Liquidus phasecels cels cels cels cels cels Liquidus Viscosity 2.81E+05 2.47E+051.09E+05 1.98E+05 2.04E+05 1.99E+05 Mol % 77 78 79 80 81 82 SiO₂ 70.0270.61 69.72 69.57 69.39 69.79 Al₂O₃ 12.86 12.59 12.50 12.36 12.22 12.37B₂O₃ 1.75 1.75 1.93 1.93 1.93 1.70 MgO 4.94 4.83 4.89 4.99 5.09 4.99 CaO4.94 4.83 5.22 5.32 5.43 5.32 SrO 1.40 1.38 1.46 1.49 1.52 1.49 BaO 3.533.45 3.71 3.78 3.86 3.78 La₂O₃ 0.00 0.00 0.00 0.00 0.00 0.00 As₂O₃ 0.000.00 0.00 0.00 0.00 0.00 Sb₂O₃ 0.47 0.47 0.49 0.48 0.48 0.48 ZrO₂ 0.020.02 0.01 0.00 0.00 0.00 SnO₂ 0.07 0.07 0.07 0.08 0.08 0.08 MgO + La₂O₃4.94 4.83 4.89 4.99 5.09 4.99 Sum(RO + La2/3O) 14.81 14.49 15.28 15.5815.90 15.58 Sum(RO + La2/3O)/Al₂O₃ 1.15 1.15 1.22 1.26 1.30 1.26 StrainPoint (° C.) 735 738 732 721 721 723 CTE (×10−7/° C.) 37.7 35.9 37.237.7 39 38.2 Density 2.626 2.615 2.638 2.650 2.648 2.652 Young's modulus(Gpa) — 81.4 — — — — specific modulus — 31.1 — — — — 200 poise T (° C.)1651 1661 1661 — — — Liquidus T (° C.) 1200 1185 1170 1170 1165 1180Liquidus phase cels Cels cels cels cels cels Liquidus Viscosity 1.81E+052.67E+05 2.80E+05 — — — Mol % 83 84 85 86 87 88 SiO₂ 69.62 70.01 69.8469.36 69.58 69.81 Al₂O₃ 12.22 12.27 12.12 12.05 12.06 12.07 B₂O₃ 1.701.70 1.70 1.75 1.50 1.25 MgO 5.09 4.95 5.05 5.35 5.33 5.32 CaO 5.43 5.285.39 5.54 5.56 5.58 SrO 1.52 1.48 1.51 1.51 1.51 1.50 BaO 3.86 3.75 3.833.96 3.98 3.99 La₂O₃ 0.00 0.00 0.00 0.00 0.00 0.00 As₂O₃ 0.00 0.00 0.000.00 0.00 0.00 Sb₂O₃ 0.48 0.48 0.48 0.39 0.39 0.39 ZrO₂ 0.00 0.00 0.000.02 0.02 0.02 SnO₂ 0.08 0.08 0.08 0.07 0.07 0.07 MgO + La₂O₃ 5.09 4.955.05 5.35 5.33 5.32 Sum(RO + La2/3O) 15.90 15.46 15.78 16.36 16.38 16.39Sum(RO + La2/3O)/Al₂O₃ 1.30 1.26 1.30 1.36 1.36 1.36 Strain Point (° C.)726 728 725 — — — CTE (×10−7/° C.) 38.1 37.7 36.9 — — — density 2.6482.634 2.633 — — — Young's modulus (Gpa) — — — — — — specific modulus — —— — — — 200 poise T (° C.) — — — — — — Liquidus T (° C.) 1175 1170 1160— — — Liquidus phase cels Cels cels — — — Liquidus Viscosity — — — — — —*Sum(RO + La2/3O) = Sum(RO + 3La₂O₃)

TABLE 2 Glass A Glass B Glass C strain point 716° C. 725° C. 667° C.Time (min) Compaction (ppm) 0 0 0 0 5 −12 −16 −43 10 −20 −23 −74 15 −26−31 −91 30 −40 −45 −137 60 −61 −62 −202 120 −94 −87 −270 180 −110 −108Not measured 240 −128 −125 −359 300 −144 −141 −383

What is claimed is:
 1. An alkali metal-free glass comprising in molepercent on an oxide basis: SiO₂ 64.0-72.0 Al₂O₃  9.0-16.0B₂O₃ >0.0-5.0   MgO 2.0-7.5 CaO 2.0-7.5 BaO 1.0-6.0

wherein: (i) the glass satisfies the relationship:1.15≤Σ(MgO+CaO+SrO+BaO)/(Al₂O₃)≤1.50, where Al₂O₃, MgO, CaO, SrO, andBaO represent the mole percents of the respective oxide components, (ii)when the glass comprises the optional component SrO, the glass satisfiesthe relationship:BaO/SrO≥2.0, where BaO and SrO represent the mole percents of therespective oxide components, (iii) the amount of any oxide in the glassother than SiO₂, Al₂O₃, B₂O₃, MgO, CaO, SrO, BaO, and La₂O₃ is less thanor equal to 2.0 mole percent, (iv) the glass has a strain point greaterthan or equal to 700° C., (v) the glass exhibits a dimensional change ofless than 30 ppm for a 5 minute heat treatment at 600° C., and (vi) theglass has a Young's modulus to density ratio greater than or equal to28.0 GPa·cm³/g.
 2. The glass of claim 1 wherein the glass has a Young'smodulus to density ratio greater than 30.0 GPa·cm³/g.
 3. The glass ofclaim 1 wherein the glass has a Young's modulus to density ratio greaterthan 32.2 GPa·cm³/g.
 4. The glass of claim 1 wherein the As₂O₃ and Sb₂O₃concentrations in the glass are each less than or equal to 0.005 molepercent.
 5. The glass of claim 1 wherein the glass has a liquidusviscosity greater than or equal to 150,000 poise.
 6. A liquid crystaldisplay substrate comprising the glass of claim
 1. 7. A method forproducing alkali metal-free glass sheets by a downdraw processcomprising selecting, melting, and fining batch materials so that theglass making up the sheets has the composition of claim 1, wherein: (a)the fining is performed without the use of substantial amounts ofarsenic; and (b) a population of 50 sequential glass sheets produced bythe downdraw process from the melted and fined batch materials has anaverage gaseous inclusion level of less than 0.10 gaseousinclusions/cubic centimeter, where each sheet in the population has avolume of at least 500 cubic centimeters.
 8. The method of claim 7wherein the downdraw process comprises a fusion draw process.
 9. Themethod of claim 7 wherein the glass making up the sheets has a liquidusviscosity greater than or equal to 150,000 poise.
 10. The method ofclaim 7 further comprising using the glass sheets as substrates forliquid crystal displays.